The present invention relates to a method of forming a resist pattern by lithography and an anti-reflective layer used in forming the resist pattern.
An anti-reflection technology, to suppress reflection of exposure light from a substrate, has been known as a peripheral technology of lithography that meets the necessary conditions of dimensional precision and resolution required of ULSI manufacture. When exposure light is reflected from the substrate, a thin-film interference occurs in a resist film, producing exposure variations in the direction of resist film thickness, called standing waves, and pattern dimension variations called multiple interferences caused by resist film thickness variations. The former degrades the resolution, while the latter deteriorates the dimensional precision. Halation, which is caused by exposure light being reflected on uneven surfaces of the substrate in diagonal directions and in random directions, poses a problem that areas that are originally intended to be shielded from exposure are exposed, making it impossible to form a desired pattern. These problems depend on the intensity of the reflected light from the substrate. The more the reflected light is reduced, the more these problems are mitigated. For this reason, growing efforts are being focused on the reduction of the reflected light from the substrate.
The anti-reflection methods may be classified largely into two groups by their working principle. One group of methods uses as an anti-reflective film a so-called photoabsorptive film with a strong capability to absorb exposure light, and the second group utilizes light interference to prevent reflection. As a representative for the former, an ARC (Anti-Reflective Coating) method is available, which applies a photoabsorptive organic film over the substrate before applying a resist. Light that has passed through the resist toward the substrate is absorbed by this photoabsorptive organic film before being reflected by the substrate surface, so that the light reflected from the substrate and returning to the resist is mitigated.
Examples of anti-reflective films of the second group include Si and TiN. The anti-reflective film of Si, SiO.sub.x N.sub.y :H,TiN, etc. is deposited over a metal such as W and Al to such a thickness that the reflected light from the resist/anti-reflective film interface and the reflected light from the anti-reflective film/substrate interface are in opposite phase with each other in order to reduce the reflection. Conventionally, these methods have been employed in reducing the reflection of light.
The ARC method is described in the Proceedings of SPIE, 1991, vol. 1463, pp. 16-29 and in Japan Patent Laid-Open No. 93448/1984. The anti-reflective film using light interference is described in Japan Patent Laid-Open No. 6540/1984 and 130481/1982, in the Proceedings of SPIE, 1994, vol.2197, pp.722-732 and in the Technical Digests of International Electron Device Meeting, 1982, pp.399-402.